My '01.5 CTD block heater is 800W, plus I have 2 150W oil pan pads and a Napa trickle charger. Never felt the need to put anything on the NV5600 even at 50 below. Anyway, at those temps nothing runs smooth for a while.
2011+ Ford 6.7 is 1000 watts, 8.3 amps at 120 volts (meter measurements agree). Owners manual says engine temperature stabilizes at 3 hours so I have a timer set for 3 hours before leaving for work.
The manual for my truck states 6 or 7 amps of power draw for my motor's block heater. I have it on a timer, so 3 hours before work it's pre-heating the engine block on my truck. By the time I go to start it, it's roughly 100 degrees in the cylinder head in an ambient temp of 14 to 20 degrees farenheit.